Transformer with two-part core for fluorescent lamp circuits



Oct. 31, 1967 D. F. CHAPMAN 3,350,605 TRANSFORMER WITH TWO-PART CORE FORFLUORESCENT LAMP CIRCUITS Filed June 9, 1965 FIG.4

l P; s!

1N VENTOR. DONALD FREDERICK CHAPMAN ATTORNEY United States Patent3,350,605 TRANSFORMER WITH TWO-PART CORE FOR FLUORESCENT LAMP CIRCUITSDonald Frederick Chapman, London, England, assignor to Thorn ElectricalIndustries Limited, London, England Filed June 9, 1965, Ser. No. 462,6382 Claims. (Cl. 315-278) ABSTRACT OF THE DISCLOSURE A circuit arrangementof the semi-resonant start type for electric discharge lamps has primaryand secondary windings which each include two coils connected in seriesand each of the four coils of the windings is wound around a dilferentone of the limbs of the two U-shaped core parts which are arranged toform a magnetic circuit with air gaps between the ends of the limbs. Thetwo primary coils may be wound around the limbs of one of the U- shapedcore parts and the two secondary coils around the limbs of the other orthe primary and secondary coils may be arranged in alternation aroundthe magnetic circuit. The leakage reactances can easily be adjusted byaltering the air gaps between the U-shaped core parts.

The present invention relates to circuit arrangements of thesemi-resonant start type for electric discharge lamps such asfluorescent tubes.

This type of circuit arrangement has a primary winding in the supplycircuit of the lamp and a secondary winding and a capacitor in seriesforming a starting circuit which, with the lamp in position, isconnected to the supply circuit through the electrodes of the lamp, theprimary and secondary windings being connected in opposition andmagnetically coupled.

FIGURE 1 of the accompanying drawings shows such a circuit and FIGURES 2and 3 show the equivalent circuits under starting and runningconditions, respectively.

In FIGURE 1 the AC. mains supply is applied across supply terminals and11. The circuit includes primary and secondary windings each of which isdivided into two coils for reasons to be explained below. The coils P1and P2 of the primary winding are connected in series in the supplycircuit between the supply terminal 10 and one side of an electrodefilament 12 at one end of a fluorescent lamp 13. The coils S1 and S2 ofthe secondary windings are connected in series with a capacitor Cbetween the other side of the electrode filament 12 and one side of anelectrode filament 14 at the other end of the lamp 13, the other side ofthe electrode filament 14 being connected to the supply terminal 11. Theprimary and secondary windings. are coupled by a magnetic core on whichthey are wound and, as indicated by the arrows in FIGURE 1, areconnected in opposition.

, In FIGURE 2 the opposing reactances of the primary and secondarywindings are represented by XP and XS and the leakage reactances by Xpand Xs. During starting current at first flows only in the circuit fromterminal 10 through Xp and XP, through electrode filament 12, throughXS, Xs and C, and through electrode filament 14 to the terminal 11. Theopposed reactances XP and Xs cancel and with appropriate values for theother elements a starting voltage is developed across Xs and C which isgreater than the voltage applied to the terminals 10 and 11. The currentflowing in this circuit heats the electrode filaments to theelectron-emissive temperature and the lamp starts under the influence ofthe applied starting voltage.

The currents through XP and XS are now no longer equal but thesereactances can be replaced by a mutual reactance XM through which thelamp current flows, as shown in FIGURE 3. The leakage reactances Xp andXs now serve, under running conditions, to limit harmonic frequenciesgenerated by the lamp being fed back into the supply. To achieve thebest results it is desirable to be able to adjust these leakagereactances when assembling the ballast unit formed by the two windingsand the magnetic core. The invention provides a novel construction whichenables this to be achieved in a simple manner.

In accordance with the invention a circuit arrangement of thesemi-resonant start type for electric discharge lamps has primary andsecondary windings which each include two coils connected in series andeach of the four coils of the windings is wound around a dilferent oneof the limbs of the two U-shaped core parts which are arranged to form amagnetic circuit with air gaps between the ends of the limbs.

The two primary coils may be wound around the limbs of one of theU-shaped core parts and the two secondary coils around the limbs of theother or the primary and secondary coils may be arranged in alternationaround the magnetic circuit.

The leakage reactances can easily be adjusted by altering the air gapsbetween the 'U-shaped core parts.

The construction of the ballast unit in one example of a circuitarrangement in accordance with the invention will now be described withreference to FIGURE 4 of the accompanying drawings, which shows theballast unit diagrammatically.

The coils P1 and P2 of the primary winding are wound around the limbs ofa laminated U-shaped core part 15 of rectangular cross-section. Thecoils S1 and S2 of the secondary winding are wound around the limbs of asimilar U-shaped core part 16. The four coils are identical coils woundfrom enamelled copper wire interleaved with paper. To simplify windingthe coils P1 and S1 are wound on a common former fitting closely overthe limbs of the two core parts 15 and 16 and the coils P2 and S2 arewound on a second former. The core parts with the coils are fitted m asheet steel case of known type with the air gaps between the ends oflimbs of the core parts set to give the desired reactances and areembedded in a potting resin in known manner.

For a -watt lamp operating on a 240 volt 5O c./s. supply the totalreactance of the primary winding was 320 ohms and that of the secondarywinding the same. The leakage reactance of the primary and secondarywindings opposition was ohms. This was used with a capacitor having acapacitance of 6.0 mfd. The total harmonic content detected in thesupply current waveform was less than 3%.

When smaller leakage reactances are required the coils P2 and S2 may beinterchanged, the coils P1 and P2 remaining connected in series and inopposition to the coils S1 and S2.

I claim:

1. A circuit arrangement of the semi-resonant start type for an electricdischarge lamp, said circuit comprising primary windings in series withthe lamp and secondary windings in shunt with the lamp each includingtwo coils connected in series and further comprising two U- shaped coreparts having limbs around which said coils are wound, said core partsbeing arranged to form a magnetic circuit with air gaps between the endsof the said limbs, and each of the four coils of said windings being 10wound around a different one of the said limbs.

2. A circuit arrangement as claimed in claim 1 in which the primarycoils are Wound around the limbs of one core part and the secondarycoils around the limbs of the other core part.

References Cited UNITED STATES PATENTS 2,170,446 8/ 1939 Edwards 315-2783,200,290 8/ 1965 Moerkens 315-278 FOREIGN PATENTS 786,067 11/ 1957Great Britain.

565,576 1/1957 Italy.

DAVID J. GALVIN, Primary Examiner.

1. A CIRCUIT ARRANGEMENT OF THE SEMI-RESONANT START TYPE FOR AN ELECTRIC DISCHARGE LAMP, SAID CIRCUIT COMPRISING PRIMARY WINDINGS IN SERIES WITH THE LAMP AND SECONDARY WINDINGS IN SHUNT WITH THE LAMP EACH INCLUDING TWO COILS CONNECTED IN SERIES AND FURTHER COMPRISING TWO USHAPED CORE PARTS HAVING LIMBS AROUND WHICH SAID COILS ARE WOUND, SAID CORE PARTS BEING ARRANGED TO FOR, A MAGNETIC CIRCUIT WITH AIR GAPS BETWEEN THE ENDS OF THE SAID LIMBS, AND EACH OF THE FOUR COILS OF SAID WINDINGS BEING WOUND AROUND A DIFFERENT ONE OF THE SAID LIMBS. 